Paperboards having improved bending stiffness and method for making same

ABSTRACT

Paperboards having improved MD and CD bending stiffness by including a paperboard binder coalescing agent to cause starch paperboard binder to coat at least some of the paperboard fibers. Also a method for preparing these improved bending stiffness paperboards by combining with an untreated paperboard fiber stream a treated paperboard fiber stream where at least some/at least a portion of the paperboard fibers are coated with a starch paperboard binder due to the coalescing action of a paperboard binder coalescing agent.

FIELD OF THE INVENTION

The present invention broadly relates to hardwood fiber-containingpaperboards having improved bending stiffness. The present inventionalso broadly relates to a method for preparing such improved bendingstiffness hardwood fiber-containing paperboards by combining with anuntreated hardwood-containing paper fiber stream a treatedhardwood-containing paperboard fiber stream wherein at least some/atleast a portion of the hardwood-containing paperboard fibers are coatedwith a starch paperboard binder due to the coalescing action of apaperboard binder coalescing agent.

BACKGROUND

An important attribute of paperboard used as packaging material is itsstiffness. High bending stiffness may be desirable in many paperboardpackaging applications. For example, bending stiffness is important infolding box container paperboards because the utility of the box dependsupon its resistance to bulging when filled. The higher the bendingstiffness of the paperboard, the more rigid will be the box made fromthat paperboard, and thus the greater the resistance of that box againstloading or crushing forces.

The ability of paperboard to resist bending during the manufacture ofpackaging, such as boxes, may also be important. Paperboard is oftenbent in converting and packaging machines. The paperboard may be bentover rolls of such machines, and thus formed to certain curvatures. If,for example, the roll diameter of the packaging machine is small, thepaperboard may be highly curved, thus imparting high tensile stresses onthe convex side of the paperboard, and high compression stresses on theconcave side. If these stresses become too high, the paperboard may bedamaged by fractures and wrinkles on the surfaces thereof, as well asreducing the bending stiffness of the paperboard.

In addition, fold-crack resistance may be important for paperboards(untreated or treated with, for example, pigment coatings,fluorochemical treatments for grease resistance, moisture barriercoatings, heat sealing coatings, extrusion coatings, etc.) which arefolded to form the packaging by influencing the functionality andappearance of the packaging made from the paperboard. Cracking at thefold of the packaging may lead to strength reduction, as well as theappearance of a visible crack at the folded surface of the packaging. Infact, bending stiffness and fold cracking are related, with a higherbending stiffness leading to a higher risk of fold cracking.

SUMMARY

According to a first broad aspect of the present invention, there isprovided an article comprising a ply of paperboard comprising:

-   -   paperboard fibers which comprise at least about 50% by weight        hardwood fibers;    -   a paperboard binder, and    -   a paperboard binder coalescing agent in an amount sufficient to        cause the starch paperboard binder to coat at least a portion of        the paperboard fibers;    -   the paperboard having:        -   a caliper of from about 8 to about 28 points; and        -   a basis weight in the range of from about 105 to about 300            lbs/3000 ft²;        -   a MD bending stiffness equal to or greater than a first            curve defined by the equation:            -   y¹=0.5297x^(2.2095), wherein x is the caliper of the                paperboard and y¹ is the MD bending stiffness in Taber                Stiffness Units; and        -   a CD bending stiffness equal to or greater than a second            curve defined by the equation:            -   y²=0.2188x^(2.2681), wherein x is the caliper of the                paperboard and y² is the CD bending stiffness in Taber                Stiffness Units.

According to a second broad aspect of the present invention, there isprovided a method for preparing a paperboard, which comprises thefollowing steps:

-   -   (a) providing a first stream of treated paperboard fibers        comprising:        -   untreated paperboard fibers comprising at least about 80% by            weight untreated hardwood fibers;        -   a starch paperboard binder in a weight ratio of starch            paperboard binder to untreated paperboard fibers of from            about 0.1:1 to about 2:1; and        -   a paperboard binder coalescing agent in an amount sufficient            to cause the starch paperboard binder to coat at least some            of the untreated paperboard fibers to provide treated            paperboard fibers;    -   (b) combining the first treated paperboard fiber stream of        step (a) with a second untreated paperboard fiber stream        comprising at least about 50% by weight untreated hardwood        fibers in a weight ratio of treated paperboard fibers to        combined treated and untreated paperboard fibers in the range of        from about 15 to about 50 lbs/ton to provide a treated        paperboard web; and    -   (c) forming the treated paperboard web of step (b) into        paperboard having:        -   a caliper of from about 8 to about 28 points;        -   a basis weight in the range of from about 105 to about 300            lbs/3000 ft²;        -   a MD bending stiffness equal to or greater than a first            curve defined by the equation:            -   y¹=0.5297x^(2.2095), wherein x is the caliper of the                paperboard and y¹ is the MD bending stiffness in Taber                Stiffness Units; and        -   a CD bending stiffness equal to or greater than a second            curve defined by the equation:            -   y²=0.2188x^(2.2681), wherein x is the caliper of the                paperboard and y² is the CD bending stiffness in Taber                Stiffness Units.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in conjunction with the accompanyingdrawings, in which:

FIG. 1 represents graphical plots of bending stiffness curves in termsof Taber Stiffness Units in both the machine direction (MD) andcross-machine direction (CD) versus caliper of various paperboards whichcompare the embodiments of the paperboards according to the presentinvention having improved (minimum) MD and CD bending stiffness valueswith MD and CD bending stiffness values for control paperboards;

FIG. 2 represents graphical plots of bending stiffness curves in termsof Taber Stiffness Units in the machine direction (MD) versus caliperwhich compare the embodiments of the paperboards according to thepresent invention having improved (maximum) MD bending stiffness values,with the MD bending stiffness values for control paperboards;

FIG. 3 represents graphical plots of bending stiffness curves in termsof Taber Stiffness Units in the cross-machine direction (CD) versuscaliper which compare the embodiments of the paperboards according tothe present invention having improved (maximum) CD bending stiffnessvalues with the CD bending stiffness values for control paperboards; and

FIG. 4 is a flowchart to illustrate an embodiment of a method forpreparing improved bending stiffness paperboards according to thepresent invention.

DETAILED DESCRIPTION

It is advantageous to define several terms before describing theinvention. It should be appreciated that the following definitions areused throughout this application.

DEFINITIONS

Where the definition of terms departs from the commonly used meaning ofthe term, applicant intends to utilize the definitions provided below,unless specifically indicated.

For the purposes of the present invention, the term “paperboard web”refers to a fibrous paper web that may be formed, created, produced,etc., from a mixture, furnish, etc., comprising paperboard fibers,paperboard stiffness strengthening agents, etc., plus any other optionalpapermaking additives such as, for example, internal and/or externalpaper sizing agents, fillers, wet-strength agents, optical brighteningagents, etc. The paperboard web may be in the form of a continuous roll,a discrete sheet, etc.

For the purposes of the present invention, the term “paperboard fibers”refers to any fibrous material which may be used in preparing a fibrouspaper web. Paperboard making fibers may include pulp (wood) fibers(e.g., softwood fibers and/or hardwood fibers), kraft fibers (e.g., pulpfibers produced by the kraft pulping process), as well as wood fibersproduced by soda, sulfite, magnefite, cold soda, NSSC, etc., pulp makingprocesses, synthetic fibers, waste paper fibers, recycled paper fibers,fibers from any of hemp, jute, ramie, flax, cotton linters, abaca, woodwaste, straw, bagasse, bamboo, sisal, etc., as well as any combinationsof such fibers.

For the purposes of the present invention, the term “ply of paperboard”refers to a single ply (layer) of a paperboard web having a caliper offrom about 8 to about 28 points, such as from about 12 to about 18points. The ply of paperboard may be in the form of a continuous roll, adiscrete sheet, a packaging material blank such as for making a box,etc.

For the purposes of the present invention, the term “softwood fibers”refers to fibrous pulps derived from the woody substance of coniferoustrees (gymnosperms) such as varieties of fir, spruce, pine, etc., forexample, loblolly pine, slash pine, Colorado spruce, balsam fir, Douglasfir, jack pine, radiata pine, white spruce, lodgepole pine, redwood,etc. North American southern softwoods and northern softwoods may beused to provide softwood fibers, as well as softwoods from other regionsof the world. Inclusion of softwood fibers tends to impart greaterbending stiffness in paperboards, but also tends to impart rougher andless smooth surfaces in such paperboard.

For the purposes of the present invention, the term “hardwood fibers”refers to fibrous pulps derived from the woody substance of deciduoustrees (angiosperms) such as birch, oak, beech, maple, eucalyptus,poplars, etc. Inclusion of hardwood fibers in paperboards tends toimpart smoother surfaces in such paperboards.

For the purposes of the present invention, the term “synthetic fibers”refers to fibers other than wood pulp fibers (e.g., other than pulpfibers) and which be made from, for example, cellulose acetate, acrylic,polyamides (such as, for example, Nylon 6, Nylon 6/6, Nylon 12,polyaspartic acid, polyglutamic acid, etc.), polyamines, polyimides,polyamides, polyacrylics (such as, for example, polyacrylamide,polyacrylonitrile, esters of methacrylic acid and acrylic acid, etc.),polycarbonates (such as, for example, polybisphenol A carbonate,polypropylene carbonate, etc.), polydienes (such as, for example,polybutadiene, polyisoprene, polynorbornene, etc.), polyepoxides,polyesters (such as, for example, polyethylene terephthalate,polybutylene terephthalate, polytrimethylene terephthalate,polycaprolactone, polyglycolide, polylactide, polyhydroxybutyrate,polyhydroxyvalerate, polyethylene adipate, polybutylene adipate,polypropylene succinate, etc.), polyethers (such as, for example,polyethylene glycol(polyethylene oxide), polybutylene glycol,polypropylene oxide, polyoxymethylene(paraformaldehyde),polytetramethylene ether(polytetrahydrofuran), polyepichlorohydrin, andso forth), polyfluorocarbons, formaldehyde polymers (such as, forexample, urea-formaldehyde, melamine-formaldehyde, phenol formaldehyde,etc.), polyolefins (such as, for example, polyethylene, polypropylene,polybutylene, polybutene, polyoctene, etc.), polyphenylenes (such as,for example, polyphenylene oxide, polyphenylene sulfide, polyphenyleneether sulfone, etc.), silicon containing polymers (such as, for example,polydimethyl siloxane, polycarbomethyl silane, etc.), polyurethanes,polyvinyls (such as, for example, polyvinyl butyral, polyvinyl alcohol,esters and ethers of polyvinyl alcohol, polyvinyl acetate, polystyrene,polymethylstyrene, polyvinyl chloride, polyvinyl pryrrolidone,polymethyl vinyl ether, polyethyl vinyl ether, polyvinyl methyl ketone,etc.), polyacetals, polyarylates, and copolymers (such as, for example,polyethylene-co-vinyl acetate, polyethylene-co-acrylic acid,polybutylene terephthalate-co-polyethylene terephthalate,polylauryllactam-block-polytetrahydrofuran, vinyl chloride, regeneratedcellulose such as viscose rayon, glass fibers, ceramic fibers,bicomponent fibers, melamine fibers (e.g., fibers obtained frommelamine-formaldehyde resin), etc.

For the purposes of the present invention, the term “bicomponent fibers”refers to fibers comprising a core and sheath configuration. The coreand sheath portions of bicomponent fibers may be made from variouspolymers. For example, bicomponent fibers may comprise a PE(polyethylene) or modified PE sheath which may have a PET (polyethyleneterephthalate) or PP (polypropylene) core. In one embodiment, thebicomponent fiber may have a core made of polyester and sheath made ofpolyethylene. Alternatively, a multi-component fiber with a PP(polypropylene) or modified PP or PE sheath or a combination of PP andmodified PE as the sheath or a copolyester sheath wherein thecopolyester is isophthalic acid modified PET (polyethyleneterephthalate) with a PET or PP core, or a PP sheath-PET core and PEsheath-PP core and co-PET sheath fibers may be employed. Variousgeometric configurations may be used for the bicomponent fiber,including concentric, eccentric, islands-in-the-sea, side-by-side, etc.The relative weight percentages and/or proportions of the core andsheath portions of the bicomponent fiber may also be varied.

For the purposes of the present invention, the term “paperboard bindercoalescing agents” refers to agents which may be added to, combinedwith, etc., paperboard fibers and starch paperboard binders to cause thestarch paperboard binders coalesce on and coat the paperboard fibers,and thus increase to the bending stiffness of the resulting paperboards.Suitable paperboard binder coalescing agents may include combinations,mixtures, etc., of cationic non-starch polymers such as cationicpolyacrylamides, cationic polyesters, cationic styrene acrylics,cationic styrene butadiene latexes, cationic polyvinyl alcohols,cationic polyvinyl acetates, etc., and cationic starches (e.g., cationicstarch paperboard binders), which have been cooked, reacted, etc., suchas, for example, Ceregel from Cerealus Holdings LLC.

For the purposes of the present invention, the term “paperboard binder”refers to paper binder agents for paper webs. Paperboard binders mayinclude synthetic or naturally occurring polymers (or a combination ofdifferent polymers), for example, starch binders, as well as non-starchbinders such as polyvinyl alcohol (PVOH), proteinaceous adhesives suchas, for example, casein or soy proteins, etc.; polymer latexes such asstyrene butadiene rubber latexes, acrylic polymer latexes, polyvinylacetate latexes, styrene acrylic copolymer latexes, etc., or anycombination thereof. Paperboard binders useful may comprise exclusively(100%) starch binders, or may comprise minimal amounts (e.g., up toabout 10%, such as up to about 1%) other non-starch binders in additionto the starch binders.

For the purposes of the present invention, the term “starch paperboardbinders” refers to paper binder agents which comprise exclusively (100%)starch, a starch derivative, etc., or any combination thereof. Suitablestarch paperboard binders may be derived from a natural starch, e.g.,natural starch obtained from a known plant source, for example, wheat,maize, potato, tapioca (e.g., pearl starch), etc. The starch paperboardbinders may be modified (i.e., a modified starch) by one or morechemical treatments known in the paper starch binder art, for example,by oxidation to convert some of OH groups to —COOH groups, etc., toformed oxidized starches. In some cases the starch paperboard binder mayhave a small proportion of acetyl groups. Alternatively, the starchpaperboard binders may be chemically treated to render them cationic(i.e., cationic starch paperboard binders) or amphoteric (i.e.,amphoteric starch paperboard binders), i.e., with both cationic andanionic charges. The starch paperboard binders may also be a starchconverted to a starch ether, or a hydroxyalkylated starch by replacingsome —OH groups with, for example, —OCH₂CH₂OH groups, —OCH₂CH₃ groups,—OCH₂CH₂CH₂OH groups, etc., e.g., ethylated starch. A further class ofchemically treated starch paperboard binders which may be used are knownas the starch phosphates. Alternatively, raw starch may be hydrolyzed bymeans of a dilute acid, an enzyme, etc., to produce starch paperboardbinders in the form of a gum of the dextrin type.

For the purposes of the present invention, the term “paperboard filler”refers to mineral products (e.g., calcium carbonate, kaolin clay, etc.),as well as nonmineral products (e.g., plastic pigments), which may beused in paperboard making to reduce materials cost per unit mass of thepaperboard, increase opacity, increase smoothness, etc. The mineralproducts may be finely divided, for example, the size range of fromabout 0.5 to about 5 microns. When included, the paperboard filler maycomprise from to about 0.1 to about 10% by weight of the paperboard,such as from about 1 to about 5% by weight of the paperboard.

For the purposes of the present invention, the term “paperboard pigment”refers to a material (e.g., a finely divided particulate matter) whichmay be used or may be intended to be used to affect optical propertiesof a paperboard. Paperboard pigments may include calcium carbonatepigments, absorptive plastic pigments, clay pigments, kaolin pigments,calcined clay pigments, talc pigments, titanium dioxide pigments, bariumsulfate pigments, silica pigments, zeolite pigments, etc. Paperboardpigments may also be platy mineral pigments, non-platy mineral pigments,etc.

For the purposes of the present invention, the term “calcium carbonate”refers various calcium carbonates which may be used as paperboardpigments, such as precipitated calcium carbonate (PCC), ground calciumcarbonate (GCC), modified PCC and/or GCC, etc.

For the purposes of the present invention, the term “precipitatedcalcium carbonate (PCC)” refers to a calcium carbonate which may bemanufactured by a precipitation reaction and which may used as apaperboard pigment. PCC may comprise almost entirely of the calcitecrystal form of CaCO₃. The calcite crystal may have several differentmacroscopic shapes depending on the conditions of production.Precipitated calcium carbonates may be prepared by the carbonation, withcarbon dioxide (CO₂) gas, of an aqueous slurry of calcium hydroxide(“milk of lime”). The starting material for obtaining PCC may compriselimestone, but may also be calcined (i.e., heated to drive off CO₂),thus producing burnt lime, CaO. Water may added to “slake” the lime,with the resulting “milk of lime,” a suspension of Ca(OH)₂, being thenexposed to bubbles of CO₂ gas. Cool temperatures during addition of theCO₂ tend to produce rhombohedral (blocky) PCC particles. Warmertemperatures during addition of the CO₂ tend to produce scalenohedral(rosette-shaped) PCC particles. In either case, the end the reactionoccurs at an optimum pH where the milk of lime has been effectivelyconverted to CaCO₃, and before the concentration of CO₂ becomes highenough to acidify the suspension and cause some of it to redissolve. Incases where the PCC is not continuously agitated or stored for manydays, it may be necessary to add more than a trace of such anionicdispersants as polyphosphates. Wet PCC may have a weak cationiccolloidal charge. By contrast, dried PCC may be similar to most groundCaCO₃ products in having a negative charge, depending on whetherdispersants have been used. The calcium carbonate may be precipitatedfrom an aqueous solution in three different crystal forms: the vateriteform which is thermodynamically unstable, the calcite form which is themost stable and the most abundant in nature, and the aragonite formwhich is metastable under normal ambient conditions of temperature andpressure, but which may convert to calcite at elevated temperatures. Thearagonite form has an orthorhombic shape that crystallizes as long, thinneedles that may be either aggregated or unaggregated. The calcite formmay exist in several different shapes of which the most commonly foundare the rhombohedral shape having crystals that may be either aggregatedor unaggregated and the scalenohedral shape having crystals that aregenerally unaggregated.

For the purposes of the present invention, the term “calenderedpaperboard” refers to a paperboard which has been subjected tocalendering to, for example, smooth out the material for enablingprinting on the material, to increase the gloss on the material surface,etc. For example, calendering may involve a process of using pressure(and optionally temperature and moisture) for embossing a smooth surfaceon the still rough material surface. Calendering may be carried out on acalender which may comprise a series of calender rolls at the end of,for example, a papermaking machine (on-line), or separate from thepapermaking machine (off-line). Calendering may includesupercalendering, hot-soft calendering, moisture-gradient calendering,extended nit calendering, belt calendering, etc. See G. A. Smook,Handbook for Pulp and Paper Technologists (2^(nd) Edition, 1992), pages273-78, the entire contents and disclosure of which is hereinincorporated by reference, for a general description of calendering, aswell as devices for carrying out calendering, that may be useful herein.

For the purposes of the present invention, the term “basis weight”refers to the grammage of a sheet, roll, etc., of material comprisingthe paperboard, with or without layers or coatings, as determined byTAPPI test T410. See G. A. Smook, Handbook for Pulp and PaperTechnologists (2^(nd) Edition, 1992), page 342, Table 22-11, the entirecontents and disclosure of which is herein incorporated by reference,which describes the physical test for measuring basis weight. The basisweight of the paperboard is essentially a measure of the density of thatpaperboard per unit area, herein reflected in units of lbs/3000 ft².Suitable basis weights for use herein are in the range of from about 105to about 300 lbs/3000 ft², such as from about 140 to about 200 lbs/3000ft².

For the purposes of the present invention, the term “caliper,” refers tothe thickness of a sheet, web, etc., of a material, for example, amaterial comprising the paper web, with or without layers or coatings,before or after calendaring, in mils, as determined by measuring thedistance between smooth, flat plates at a defined pressure.

For the purposes of the present invention, the term “mil(s)” is used inthe conventional sense of referring to thousandths of an inch and isalso referred to interchangeably herein as “points.”

For the purposes of the present invention, the term “MD” refers tomachine direction of the paperboard, i.e., is used in the conventionalpapermaking sense of the direction the paperboard moved during itsformation.

For the purposes of the present invention, the term “CD” refers to thecross-machine direction, i.e., is used in the conventional papermakingsense of the direction transverse to the machine direction (MD).

For the purposes of the present invention, the term “bending stiffness”(also referred to interchangeably herein as “bending resistance” refersto flexural rigidity of the paperboard, especially the specific flexuralrigidity. Bending stiffness depends upon both the modulus of elasticityof and thickness of the paperboard board. Bending stiffness is measuredherein in terms of Taber Stiffness Units in either the machine direction(MD) or cross-machine direction (CD).

For the purposes of the present invention, the term “flexural rigidity”refers to a measure of the stiffness of a paper strip, such as a stripof paperboard, in terms of its width (breadth), tensile modulus orelastic modulus (i.e., Young's modulus), and caliper (thickness).

For the purposes of the present invention, the term “specific flexuralrigidity” refers to the flexural rigidity of the paperboard in terms ofits width (breadth).

For the purposes of the present invention, “Taber Stiffness Units” aredefined as the bending moment of ⅕ of a gram applied to a 1.5″ widespecimen of paperboard at a 5 centimeter test length, flexing it to anangle of 15°. A Taber Stiffness Unit is the equivalent of one gramcentimeter. The method used herein for measuring Taber Stiffness isTAPPI T566 (Bending Resistance (Stiffness) of Paper).

For the purposes of the present invention, the term “Huygen Bond” refersto the degree of internal bonding of the paperboard/paperboard fibersand is measured in units of ft. lbs/in². The Huygen Bond values of apaperboard may be measured in the machine direction (MD), as well as thecross-machine (CD) direction by using TAPPI T569 om-99 (Internal BondStrength (Scott Type)).

For the purposes of the present invention, the term “bulk” refers to thevolume or thickness of the paperboard in relation to its weight. Bulk isthe reciprocal of the density (weight per unit volume), and may becalculated from caliper and basis weight of the paperboard. Decreasingthe bulk (or in other words, increasing the density) of, for example, asheet of paperboard, causes that sheet to be smoother, glossier, lessopaque, darker, lower in strength, etc.

For the purposes of the present invention, the term “solids basis”refers to the weight percentage of each of the respective solidmaterials (e.g., paperboard fibers, paperboard stiffness strengtheningagents, paperboard pigments, etc.) present in the composition, etc., inthe absence of any liquids (e.g., water, other solvents, etc.). Unlessotherwise specified, all percentages given herein for the solidmaterials are on a solids basis.

For the purposes of the present invention, the term “lbs/ton” refers tothe amount (lbs) of paperboard stiffness strengthening agent relative tothe amount (ton) of paperboard fibers.

For the purposes of the present invention, the term “solids content”refers to the percentage of non-volatile, non-liquid components (byweight) that are present in the composition, etc.

For the purpose of the present invention, the term “applying” withreference to the coatings, and compositions used to provide suchcoatings, may include adding, depositing, spraying, daubing, spreading,wiping, dabbing, dipping, printing, etc.

For the purposes of the present invention, the term “Parker PrintSmoothness” refers to the extent to which the paper surface deviatesfrom a planar or substantially planar surface, as affected by the depthof the paper, paper width, numbers of departure from that planarsurface, etc., as measured by TAPPI test method T 555 om-99 at aclamping pressure of 10 kgf/cm². Parker Print Smoothness values reflectthe degree of “microroughness” of the paperboard or coating surface. Thehigher the Parker Print Smoothness value, the rougher the paperboard, orcoating surface thereof. Conversely, the lower Parker Print Smoothnessvalue, the smoother the paperboard, or coating surface thereof. For theembodiments of the paperboards of the present invention, the ParkerPrint Smoothness values may be about 3 or less, such as about 2 or less,such as in the range of from about 1 to about 1.5.

For the purposes of the present invention, the term “Solid BleachedSulfate (SBS)” refers to a U.S. grade of bleached paperboard coated onone or both sides with a paperboard pigment. The corresponding Europeangrade of SBS is SBB/GZ (wherein G=coated and Z=bleached virgin chemicalpulp), and the corresponding Asian grade is Ivoryboard.

For the purposes of the present invention, the term “Coated UnbleachedKraft (CUK)” refers to a U.S. grade of kraft unbleached paperboardcoated on one or both sides with a paperboard pigment. The correspondingEuropean grade of CUK is coated SUB/GN (wherein G=coated andN=unbleached virgin chemical pulp).

For the purposes of the present invention, the term “Multi-Ply Recycled(MPP)” refers to a U.S. grade of pigment coated multi-ply paperboardwith recycled inner layers (plies) and bleached outer layer(s) (plies).The corresponding European grade of MPP is coated GD/GT (whereinG=coated, D=recycled with grey back, and T=recycled with white or creamback), and the corresponding Asian grade is Coated Duplex-Greyback,Whiteback, and White Lined Chip (WLC).

For the purposes of the present invention, the term “Coated RecycledBoard (CRB)” refers to a U.S. grade of recycle paperboard coated on oneside.

For the purposes of the present invention, the term “Uncoated RecycledBoard (URB)” refers to a U.S. grade of uncoated recycled paperboard. Thecorresponding European grades of URB are chipboard, coreboard, gypsumwallboard facing paper, etc., and the corresponding Asian grades aregreyboard, coreboard, gypsum wallboard facing paper, etc.

For the purposes of the present invention, the term “Folding Boxboard(European Style FFB)” refers to a U.S. grade of pigment coated multi-plypaperboard with a mechanical pulp inner layer (ply). The correspondingEuropean grade of Folding Boxboard is FBB/FCI (wherein C=virginmechanical pulp), and the corresponding Asian grade is Ivoryboard.

For the purposes of the present invention, the term “liquid” refers to anon-gaseous fluid composition, compound, material, etc., which may bereadily flowable at the temperature of use (e.g., room temperature) withlittle or no tendency to disperse and with a relatively highcompressibility.

For the purposes of the present invention, the term “room temperature”refers to the commonly accepted meaning of room temperature, i.e., anambient temperature of 20° to 25° C.

For the purposes of the present invention, the term “wet end” refers tothat portion of the papermaking process involving an aqueous slurry ofpaper fibers, fillers, other additives (e.g., wet strength agents), etc.The wet end of papermaking often deals with the interactions betweenpaper furnish materials and the chemical/chemical physical processeswhich occur at the wet end of the papermaking machine. See G. A. Smook,Handbook for Pulp and Paper Technologists (2^(nd) Edition, 1992), pages219-21, the entire contents and disclosure of which is hereinincorporated by reference, for a general description of wet endchemistry that occurs during papermaking, such as paperboard making.

DESCRIPTION

One of the primary functions of packaging is to protect the productwithin that packaging. One such function is the ability of the package(e.g., a box made from folded paperboard) to withstand the forcesencountered during distribution, display, etc., of that package. Thepackage will desirably remain closed (unopened) and will also protectthe product inside of the package. During shipping, some of the primarymodes of abuse of packaged products are vibrations during transport, forexample, as may occur during transport in trucks or rail cars and/or bydropping of the packaged product. Also during shipping of the packagedproduct, a majority of the abuse of a packaged product may occur in thevertical direction as cases of the packaged product are bounced boutwhen, for example, the truck or rail car encounters (strikes)obstructions in the vehicles path. Assuming that the packaged product(s)remain in an “end up” configuration, most, but not all, of the droppingof the packaged product may thus occur in the vertical direction.

These vertical impacts may result in the compressive forces beingapplied to the package. For example, many packaged products arerectangular in configuration with the longest axis of the packagedproduct being in the vertical direction. Accordingly, the packagedproduct (and any such abuse thereof) may be modeled as a rectangularcolumn. Disregarding damage which may be caused by punctures or cutting,the primary mode of damage may be due to buckling of the sidewalls ofthe package containing the product. In this mode, the physical strengthof the package can be assumed to follow the Euler buckling equation (1):P=(n×π ² ×E×I)/L ²  (1)wherein P=the allowable load (in lbs.), n=a factor accounting for theend conditions, E=the modulus of elasticity (in lb./in² or kPa), L=thelength of column (in inches or meters), and I=the moment of inertia (inin⁴ or mm⁴).

Relative to the factor accounting for end conditions (n), for a columnpivoted at both ends, n=1. For a column having one end fixed, and theother end round, n=2. For a column having both ends fixed, n=4. For acolumn having one fixed end and one free end, n=0.25.

From the Euler buckling equation (1) above, it can be seen thatincreasing the modulus of elasticity (E) also increases the allowableload (P) that the column may withstand before buckling. While manypackages may be shorter than what would be considered a long slendercolumn, nonetheless, the application of a vertical force to the packagetends to produce failure modes similar to such long columns, wherein oneor more panels of the package may buckle.

Besides the Euler buckling equation (1) above, one of the best measuresof bending stiffness of a material such as paperboard is a parametergenerally referred to as “flexural rigidity.” Flexural rigidity providesa measure of the stiffness of a paper strip, such as a strip ofpaperboard which may be folded to form a package. In fact, stiffness mayoften be most important property in folding box paperboards because theutility of the box as a package may depend upon its resistance tobulging, buckling, etc., when filled with a product.

Flexural rigidity (FR) of may be defined by the equation (2):FR=( 1/12)×b×E×t ³  (2)wherein b is the width (breadth) of the paper strip, E is Young'smodulus of the paper strip also known as the tensile modulus or elasticmodulus which is a measure of the stiffness of an elastic material andis defined by the ratio of the stress along an axis over the strainalong that axis in the range of that stress), and t is the caliper ofthat paper.

A closely related quantity to flexural rigidity (FR) is the specificflexural rigidity (SPR), which may be defined by the equation (3):SPR=FR/b= 1/12  (3)

What specific flexural rigidity (SPR) does is define the flexuralrigidity (FR) in terms of the width of the paper strip (e.g.,paperboard). In fact, the caliper t (thickness) of paper, such aspaperboard, has a very strong effect on its bending stiffness, as shownabove by equations (2) and (3). For example, doubling the caliper ofpaper or paperboard means an eightfold increase in bending stiffness.Because paperboards tend to have a higher (greater) caliper t(thickness) than paper, the thicker paperboard tends to be much stifferthan paper. Accordingly, the bending stiffness in paperboards haspreviously been achieved by, for example, increasing the caliper of thepaperboard. Unfortunately, increasing the caliper of paperboard alsogenerally causes an increase in basis weight of the paperboard, as basisweight and caliper (thickness) are linearly related (especially for agiven papermaking machine and grade of paperboard). A higher basisweight paperboard creates a heavier package, causing an increase theshipping weight of the packaged product, and thus an increase inshipping costs of the packaged product.

Improved bending stiffness may also be imparted to the paperboard bysimply increasing the density (densifying) of the paperboard. Suchdensification also decreases the caliper (thickness) of the paperboard.But densification of the paperboard to increase bending stiffness maycause other disadvantages such as difficulty scoring the sheet, damageto the sheet surface due to crushing, reduction in tensile strength,etc.

Improved bending stiffness may also be imparted to singly plypaperboards by increasing the content of softwood fibers. Unfortunately,the potential disadvantage in increasing the softwood fiber content ofpaperboards is increased roughness and decreasing smoothness of thesurface of the paperboard which makes printing on the surface of suchpaperboards more difficult and less optimum. To compensate for suchsurface roughness, multi-ply paperboards having an interior paperboardply comprising primarily softwood fibers for imparting bending stiffness(or ground wood fibers for increased bulk), and outer plys comprisingprimarily hardwood fibers for improved surface smoothness. Increasingthe bulk (e.g., by inclusion of ground wood fibers results in greaterthickness which increases the bending stiffness. But multi-plypaperboards may suffer from delamination between the respective plieswhen the multi-ply paperboard is scored, bent, etc.

By contrast, embodiments of the paperboard of the present inventionachieve improved bending stiffness in single paperboard ply, yet whileessentially maintaining the basis weight of the paperboard, as well asincreasing the content of hardwood paperboard fibers for increasedsmoothness benefits. For example, embodiments of the present inventionenable paperboards comprising increased hardwood paperboard fibercontent to be manufactured which have reduced caliper (e.g., areductions in caliper of as much as about 2 points), yet maintain atleast equivalent bending stiffness to paperboards having the highercalipers, as well as providing the surface smoothness of paperboardplies comprising higher contents of hardwood fibers. These bendingstiffness/surface smoothness benefits in the single ply of paperboardembodiments of the present invention may be achieved by improving fiberto fiber bonding throughout the paperboard by utilizing paperboardstiffness strengthening agents, along with of other paperboard binders(e.g., starch) in a portion of the hardwood paperboard fiber stream toprovide at least some treated hardwood paperboard fibers.

Embodiments of the article comprising improved bending stiffnesspaperboard comprise a ply of paperboard that includes: paperboard fiberscomprising at least about 50% by weight hardwood fibers (and up to 100%by weight hardwood fibers, for example, from about 60 to about 95% byweight hardwood fibers, such as from about 65 to about 80% by weighthardwood fibers, the balance, if any, being softwood fibers); a starchpaperboard binder (e.g., cationic starch or oxidized); and paperboardstiffness strengthening agent in an amount sufficient (for example, atleast about 0.1% by weight of the starch paperboard binder, such as fromabout 0.3 to about 1% by weight of the starch paperboard binder) tocause the starch paperboard binder to coalesce on and coat (partially orcompletely) at least some/at least a portion of the paperboard fibers.The paperboard has: (a) a caliper of from about 8 to about 28 points(such as from about 12 to about 18 points); (b) a basis weight in therange of from about 105 to about 300 lbs/3000 ft² (such as from about140 to about 200 lbs/3000 ft²); (c) a MD bending stiffness equal to orgreater than a first curve defined by equation (4), y¹=0.5297x^(2.2095),wherein x is the caliper of the paperboard and y¹ is the MD bendingstiffness in Taber Stiffness Units; and (D) a CD bending stiffness equalto or greater than a second curve defined by equation (5),y²=0.2188x^(2.2681), wherein x is the caliper of the paperboard and y²is the CD bending stiffness in Taber Stiffness Units. (See curves MD-1and CD-1 in FIG. 1 as discussed below.) In some embodiments, thepaperboard has: (c) a maximal MD bending stiffness no greater than athird curve defined by equation (6), y¹=0.7949x^(2.2095), wherein x isagain the caliper of the paperboard and y¹ is again the MD bendingstiffness in Taber Stiffness Units; and (d) a maximal CD bendingstiffness no greater than a fourth curve defined by equation (7),y²=0.3282x^(2.2095), wherein x is again the caliper of the paperboardand y² is again the CD bending stiffness in Taber Stiffness Units. (Seecurves MD-2 and CD-2 in FIGS. 2 and 3 as discussed below.) Someembodiments of the paperboards of the present invention may also have aParker Print Smoothness value of about 3 or less, such as about 2 orless, e.g., in the range of from about 1 to about 1.5.

In some embodiments of the present invention, a multi-ply paperboardproduct may be formed which comprises a ply of the improved bendingstiffness paperboard as a first interior ply, the first interior plyhaving a first side and second side. The multi-ply paperboard productalso comprises an additional exterior ply or plies of a differentpaperboard(s) positioned on, disposed on, adhered to, etc., one or bothof the first and second sides, i.e., may be formed as a multi-plypaperboard product. In other embodiments of the present invention, asingle ply paperboard product may be formed which consists essentiallyof a single ply of the improved bending stiffness paperboard.

Embodiments of the improved bending stiffness paperboards of the presentinvention may be prepared by providing a first stream of treatedpaperboard fibers comprising: untreated paperboard fibers comprising atleast about 80% by weight untreated hardwood paper fibers (and up to100% by weight untreated hardwood paper fibers, for example from about85 to 100% weight hardwood fibers, such as from about 90 to 100% byweight hardwood fibers, the balance, if any, being softwood fibers); astarch paperboard binder in a weight ratio of starch paperboard binderto untreated paperboard fibers of from about 0.1:1 to about 2:1 (such asfrom about 0.5:1 to about 1.5:1); and a paperboard binder coalescingagent in an amount sufficient to cause the starch paperboard binder tocoalesce and coat (partially or completely) at least some of thepaperboard fibers to provide treated paperboard fibers. The combinationof untreated paperboard fibers, starch paperboard binder, and paperboardbinder coalescing agent may be heated, cooked, reacted, etc., attemperatures in the range of from about 150° to about 212° F., such asfrom about 170° to about 190° F., for up to about 5 minutes (such as forup to about 2 minutes) to form the first treated paperboard fiberstream. This first treated paperboard fiber stream is combined with asecond untreated paperboard fiber stream comprising at least about 50%by weight untreated hardwood fibers (and up to 100% by weight untreatedhardwood fibers, for example, from about 60 to about 95% by weightuntreated hardwood fibers, such as from about 65 to about 80% by weightuntreated hardwood fibers) in a weight ratio of treated paperboardfibers to combined treated and untreated paperboard fibers in the rangeof from about 15 to about 50 lbs/ton (such as in the range of from about20 to about 40 lbs/ton) to provide a treated paperboard web. Thistreated paperboard web is then formed into the improved bendingstiffness paperboard having the caliper values and basis weight valuesdescribed above, as well as the at least the minimum MD bending and CDstiffness values as described above, i.e., equal to or greater than thefirst and second curves defined, respectively, by equations (4) and (5)above, and in some embodiments, having the maximal MD bending and CDstiffness values as described above i.e., no greater than the third andfourth curves defined, respectively, by equations (6) and (7) above.

FIG. 1 represents graphical plots, indicated generally as 100, ofvarious paperboards in terms of bending stiffness (in Taber StiffnessUnits) in both the machine direction (MD) and cross-machine direction(CD) versus caliper (in points, also equivalent to mils). Points for therespective curves are plotted at calipers of 12, 14, 16, 18, 20, 22, 24,26, and 28. Curves MD-C and CD-C represent, respectively, the graphicalplots of MD and CD bending stiffness values versus caliper of thecontrol paperboards where the curve MD-C is based on the equation (8),c¹=1576x^(2.5239), wherein x is the caliper of the paperboard and c¹ isthe MD bending stiffness in Taber Stiffness Units, and where curve CD-Cis based on the equation (9), c²=0.0585x^(2.6209), wherein x is thecaliper of the paperboard and c² is the CD bending stiffness in TaberStiffness Units. By contrast, curves MD-1 and CD-1 represent,respectively, graphical plots of the MD and CD bending stiffness valuesversus caliper for embodiments of the paperboards of the presentinvention having improved (minimum) MD and CD bending stiffness values,where curve MD-1 involves on plotting caliper (x) values based onequation (4) above, and where curve CD-1 involves plotting caliper (x)values based on equation (5) above. As be seen in FIG. 1, theembodiments of the paperboards of the present invention have higher(improved) MD and CD bending stiffness values, relative to the MD and CDbending stiffness values of the control paperboards having the samecaliper (thickness).

FIG. 2 represents graphical plots, indicated generally as 200, of thebending stiffness curves in terms of Taber Stiffness Units in themachine direction (MD) versus caliper which compare embodiments for thepaperboards according to the present invention having improved (maximum)MD bending stiffness values, with MD bending stiffness values forcontrol paperboards. Curve MD-C (see also FIG. 1) represents thegraphical plot of the MD bending stiffness values versus caliper of thecontrol paperboards. By contrast, curve MD-2 represents a graphical plotof the bending stiffness values versus caliper for the embodiments ofthe paperboards of the present invention having improved (maximum) MDbending stiffness values. Curve MD-2 involves plotting of caliper (x)values based on equation (6) above.

FIG. 3 represents graphical plots, indicated generally as 300, of thebending stiffness curves in terms of Taber Stiffness Units in thecross-machine direction (CD) versus caliper which compare embodimentsfor the paperboards according to the present invention having improved(maximum) CD bending stiffness values, with the MD bending stiffnessvalues for control paperboards. Curve CD-C (see also FIG. 1) representsthe graphical plot of the CD bending stiffness values versus caliper ofthe control paperboards. By contrast, curve CD-2 represents a graphicalplot of the CD bending stiffness values versus caliper for theembodiments of the paperboards of the present invention having improved(maximum) CD bending stiffness values. Curve CD-2 involves plottingcaliper (x) values in equation (7) above.

FIG. 4 represents a flowchart to illustrate an embodiment of a methodfor preparing improved bending stiffness paperboards according to thepresent invention, which is generally indicated as 400. In one step ofmethod 400, as source of Paperboard Fibers (comprising at least about80% by weight, such as at least about 90% by weight, and up to andincluding 100% by weight, hardwood fibers), indicated as 402, a sourceof Starch (Paperboard) Binder, indicated as 404, and as a source of aPaperboard Stiffness Strengthening Agent/[Adhesion Promoter] arecombined together (appropriate amounts, as described above), asindicated by arrows 408, 410, and 442, and then heated, cooked, reacted,etc., at a temperature in the range of from about 150° to about 212° F.(such as from about 170° to about 190° F.) for up to about 5 minutes(such as up to about 2 minutes) to form a Treated Fiber Stream,indicated as 410.

In the next step, an Untreated Fiber Stream (e.g., comprising at leastabout 50% by weight, and including up to 100% by weight hardwood fibers,such as from about 65 to about 80% by weight hardwood fibers), indicatedby 416, is combined together with this Treated Fiber Stream 410, asindicated by arrows 418 and 420, in appropriate weight ratios (asdescribed above), and is then deposited on, for example, a paperboardforming wire, to provide a Paperboard Web, as indicated as 422.Paperboard Web 422. Paperboard Web 434 may then be further processed,e.g., calendered, dried, applying aqueous barrier coatings, pigmentedcoatings, extrusion coatings, or laminations, etc., as indicated byarrow 424, to provide a Paperboard, as indicated by 426, having thecaliper values, basis weight values, and improved MD and CD bendingstiffness values (as described above) according to embodiments of thepresent invention.

EXAMPLES

Paperboards having a caliper of about 16.5 points and a basis weight inthe range are formed from paperboard fiber streams comprising 100% byweight hardwood fibers. For the Control (C) Paperboard, no starchpaperboard binder is used in preparing the paperboard. Three samples(S-1, S-2, and S-3) of paperboards are also prepared by combining atreated hardwood fiber stream and an untreated hardwood fiber stream atvarying weight ratios (lbs/ton) of treated hardwood fibers to combinedtreated and untreated hardwood fibers. The treated hardwood fiber streamis prepared from a hardwood fibers, cationic starch as the paperboardbinder in about a 1:1 weight ratio of starch to hardwood fibers, andabout 0.7% (by weight of the starch) of Ceregel A. This mixture ofhardwood fibers, starch, and Ceregel is cooked at about 170°-190° F. for2 minutes to provide the treated hardwood fiber stream before beingcombined with the untreated hardwood fiber stream to prepare the S-1,S-2, and S-3 Paperboards.

The properties of the Control (C) Paperboard, as well as the S-1, S-2,and S-3 Paperboards are shown in Table 1 below:

TABLE 1 Control Sample Sample Sample (C) S-1 S-2 S-3 Caliper (points)16.5 16.5 16.5 16.5 Basis Weight 165 165 165 165 (lbs/3000 ft²) MDBending 284.0 266.6 296.6 344.2 Stiffness (Taber Stiffness Units)¹ CDBending Stiffness 115.8 115.0 129.8 148.2 (Taber Stiffness Units)¹lbs/ton¹ 0 10 20 50 ¹g-cm. ²lbs (treated fiber) per ton (treated +untreated fiber)

The results from Table 1 show is an initial decrease in bendingstiffness (compare S-1 to C), followed by an increase in bendingstiffness (compare S-2 and S-3 to C).

All documents, patents, journal articles and other materials cited inthe present application are hereby incorporated by reference.

Although the present invention has been fully described in conjunctionwith several embodiments thereof with reference to the accompanyingdrawings, it is to be understood that various changes and modificationsmay be apparent to those skilled in the art. Such changes andmodifications are to be understood as included within the scope of thepresent invention as defined by the appended claims, unless they departtherefrom.

What is claimed is:
 1. An article comprising a ply of paperboardcomprising: paperboard fibers which comprise at least about 50% byweight hardwood fibers; a starch paperboard binder, and a paperboardbinder coalescing agent in an amount sufficient to cause the starchpaperboard binder to coat at least a portion of the paperboard fibers;the paperboard having: a caliper of from about 12 to about 18 points; abasis weight in the range of from about 140 to about 200 lbs/3000 ft²; aMD bending stiffness equal to or greater than a first curve defined bythe equation: y¹=0.5297x^(2.2095), wherein x is the caliper of thepaperboard and y¹ is the MD bending stiffness in Taber Stiffness Units;and a CD bending stiffness equal to or greater than a second curvedefined by the equation: y²=0.2188x^(2.2681), wherein x is the caliperof the paperboard and y² is the CD bending stiffness in Taber StiffnessUnits.
 2. The article of claim 1, wherein the paperboard bindercoalescing agent comprises the cooked combination of a cationicnon-starch polymer and a cationic starch in an amount of at least about0.1% by weight of the starch paperboard binder.
 3. The article of claim2, wherein the paperboard coalescing agent comprises the cookedcombination of a cationic polyacrylamide and a cationic starch in anamount of from about 0.3 to about 1% by weight of the starch paperboardbinder.
 4. The article of claim 1, wherein the paperboard has: a MDbending stiffness no greater than a third curve defined by the equation:y¹=0.7949x^(2.2095); and a CD bending stiffness no greater than a fourthcurve defined by the equation: y²=0.3282x^(2.2095).
 5. A multi-plypaperboard product which comprises a ply of the paperboard of claim 1 asa first interior ply having a first side and second side, and at leastone additional different paperboard as an exterior ply positioned on oneor both of the first and second sides.
 6. The article of claim 1,wherein the starch paperboard binder comprises one or more of: cationicstarch, oxidized starch, pearl starch, or ethylated starch.
 7. Thearticle of claim 1, wherein the paperboard fibers comprise from about 60to about 95% by weight hardwood fibers.
 8. The article of claim 1,wherein the paperboard has a Parker Print Smoothness value about 3 orless.
 9. A method for preparing a paperboard, which comprises thefollowing steps: (a) providing a first stream of treated paperboardfiber stream comprising: untreated paperboard fibers comprising at leastabout 80% by weight untreated hardwood fibers; a starch paperboardbinder in a weight ratio of starch paperboard binder to untreatedpaperboard fibers of from about 0.1:1 to about 2:1; and a paperboardbinder coalescing agent in an amount sufficient to cause the paperboardbinder to coat at least some of the untreated paperboard fibers toprovide treated paperboard fibers wherein said paperboard bindercoalescing agent comprises the cooked combination of a cationicnon-starch polymer and a cationic starch in an amount of at least about0.1% by weight of the starch paperboard binder; (b) combining the firsttreated paperboard fiber stream of step (a) with a second untreatedpaperboard fiber stream comprising at least about 50% by weightuntreated hardwood fibers in a weight ratio of treated paperboard fibersto combined treated and untreated paperboard fibers in the range of fromabout 15 to about 50 lbs/ton to provide a paperboard web; and (c)forming the paperboard web of step (b) into paperboard having: a caliperof from about 8 to about 28 points; a basis weight in the range of fromabout 105 to about 300 lbs/3000 ft²; a MD bending stiffness equal to orgreater than a first curve defined by the equation: y¹=0.5297x^(2.2095),wherein x is the caliper of the paperboard and y¹ is the MD bendingstiffness in Taber Stiffness Units; and a CD bending stiffness equal toor greater than a second curve defined by the equation:y²=0.2188x^(2.2681), wherein x is the caliper of the paperboard and y²is the CD bending stiffness in Taber Stiffness Units.
 10. The method ofclaim 9, wherein the first treated paperboard fiber stream of step (a)comprises starch paperboard binder in a weight ratio of starchpaperboard binder to untreated paperboard fibers of from about 0.5:1 toabout 1.5:1.
 11. The method of claim 9, wherein the paperboard web ofstep (b) is formed in step (c) into paperboard having a caliper of fromabout 12 to about 18 points and a basis weight in the range of fromabout 140 to about 200 lbs/3000 ft².
 12. The method of claim 9, whereinthe paperboard web of step (b) is formed in step (c) into paperboardhaving: a MD bending stiffness no greater than a third curve defined bythe equation: y¹=0.7949x^(2.2095); and a CD bending stiffness no greaterthan a fourth curve defined by the equation: y²=0.3282x^(2.2095). 13.The method of claim 9, wherein the paperboard web of step (b) is formedin step (c) into paperboard having a Parker Print Smoothness value ofabout 3 or less.